This invention relates to a period measurement system, particularly of a type used to measure the period of a fetal heartbeat signal by means of an autocorrelation method.
It is conventional practice to measure the period of a biosignal, particularly of a heartbeat signal, by measuring peak spacing through application of a peak trigger system. This system finds the period of a heartbeat signal by detecting the signal peaks P.sub.1, P.sub.2, P.sub.3 . . . , and then by measuring the time between two adjacent peaks as illustrated in FIG. 1. With the peak trigger system of measurement, however, there is the possibility of measurement errors if the signal is a fetal doppler signal having a plurality of peaks within one period, or if the fetal signal has a large noise component that gives rise to a number of peaks within one period. For example, in a case where the peak trigger system is used to measure the period of a biosignal comprising two different signals S.sub.1, S.sub.2 that are generated in an alternating manner, as shown in FIG. 2, there is the possibility that the period between the mutually different signals will be detected as being the periods T.sub.1, T.sub.2 of the biosignal. In this case the trigger system would fail to measure the period accurately. Or, as depicted in FIG. 3, period measurement errors may occur due to trigger misses caused by a large noise component.
A period measurement system based on a biosignal autocorrelation method has been developed as a replacement for the peak trigger system having the defects described above. The autocorrelation system operates by sampling a heartbeat signal over a suitable sampling period, computing the autocorrelation function of the heartbeat signal on the basis of the sampled data, and measuring the period of the heartbeat signal from the computed autocorrelation function. The autocorrelation function indicates the similarity between two portions of the heartbeat signal waveform at two different times separated by a certain time interval. In other words, it represents the degree of similarity of the repeating heartbeat signal waveform. This can be better understood from FIG. 4, wherein it is seen that if a portion M.sub.1 which repeats at a certain period T is shifted along the time axis by an interval of time which is equal to the period T, the portion M.sub.1 will be superimposed on the immediately succeeding portion M.sub.2 with maximum accuracy.
In order to obtain the autocorrelation function from the biosignal, we may write the autocorrelation function .phi.(.tau.) in terms of the biosignal f(t) which is a function of the time t. Thus, .phi.(.tau.) may be written as ##EQU1##
If we let f(k) (where k=1, 2, . . . , n) denote the data obtained by sampling the signal being measured, then equation (1) shown above can be expressed as ##EQU2## expanding equation (2) gives us ##EQU3## Specifically, this is an expression showing that .phi.(.tau.) is obtained by summing the products of two items of data which exist at two different points in time separated by the phase difference variable .tau..
In equations (1), (2) and (3), .tau. represents an interval from a certain time on the heartbeat signal to a point displaced from said first point by a certain time. In other words, .tau. is a variable which applies a phase difference to the biosignal f(t), and it varies over a range which may be considered as one period of the signal.
Let us consider a common case in which the heartbeat signal of a fetus is measured to determine its period by means of the correlation method. Measurement starts by sampling the heartbeat signal at a predetermined sampling period. It is known from clinical tests that the period of a fetal heartbeat signal may cover a very wide range of from almost 300 to 1,500 milliseconds. In conventional practice, therefore, .tau. is varied over a range of from 300 to 1,500 milliseconds when conducting measurements. Since .tau./T.sub.s is employed instead of .tau. when sampling is actually conducted, .tau. is varied over a range of from 300/T.sub.s to 1,500/T.sub.s, where T.sub.s stands for the sampling period. Since the autocorrelation function found over this range of values will have a peak when .tau. is the period T of the heartbeat signal and when .tau. is an integral multiple of the period, i.e., 2T, 3T . . . , the period of the heartbeat signal can be found by detecting the peak corresponding to the period T. In the case of a fetus, however, the maximum change in the heart-rate is within .+-.15 BPM (beats per minute). Computing the autocorrelation function over a wide range as in the prior art method is therefore an essentially meaningless operation and it wastefully prolongs the time necessary for signal processing. This latter point is particularly undesirable in a period measurement system where real-time processing is strongly desired. Furthermore, conducting measurements over a meaningless wide range increases the chance that noise will influence the measurements. In addition, since the period of the fetal heartbeat signal ranges from 300 to 1,500 milliseconds, it is necessary to set the sampling period to such a value as will not diminish the accuracy of the measurement data in order to reduce the cost of the measuring apparatus and to permit period measurement processing to proceed on a real-time basis.